Oxide permanent magnet materials include hexagonal strontium ferrite and barium ferrite. Currently, strontium or barium ferrites of the magnetoplumbite type (M type) are mainly used, and they are manufactured into sintered magnets and bonded magnets.
Of magnet properties, a remanence or residual magnetic flux density (Br) and an intrinsic coercivity (HcJ) are most important.
The Br of a magnet is determined by the density and the degree of orientation of the magnet, and the saturation magnetization (4.pi.Is) which is determined by its crystal structure, and expressed by the equation:
Br=4.pi.Is.times.degree of orientation x density. The strontium and barium ferrites of the M type have a 4.pi.Is value of about 4.65 kG. The density and the degree of orientation have upper limits of about 98% even in the case of sintered magnets affording the highest values. Therefore, the Br of these magnets is limited to about 4.46 kG. It was substantially impossible in the prior art to achieve Br values of 4.5 kG or higher. PA1 A is at least one element selected from the group consisting of strontium, barium, calcium and lead, with strontium being essentially contained in A, PA1 R is at least one element selected from the group consisting of bismuth and rare earth elements inclusive of yttrium, with lanthanum being essentially contained in R, and PA1 M is cobalt or cobalt and zinc, the proportions in total of the respective elements relative to the quantity of the entire metal elements are PA1 A is at least one element selected from the group consisting of strontium, barium, calcium and lead, with strontium being essentially contained in A, PA1 R is at least one element selected from the group consisting of bismuth and rare earth elements inclusive of yttrium, with lanthanum being essentially contained in R, and PA1 M is cobalt or cobalt and zinc, PA1 A is at least one element selected from the group consisting of strontium, barium, calcium and lead, with strontium being essentially contained in A, PA1 R is at least one element selected from the group consisting of bismuth and rare earth elements inclusive of yttrium, with lanthanum being essentially contained in R, and PA1 M is cobalt or cobalt and zinc, PA1 A is at least one element selected from the group consisting of strontium, barium, calcium and lead, with strontium being essentially contained in A, PA1 R is at least one element selected from the group consisting of bismuth and rare earth elements inclusive of yttrium, with lanthanum being essentially contained in R, and PA1 M is cobalt or cobalt and zinc,
The inventors found in JP-A 115715/1997 that the inclusion of appropriate amounts of La and Zn enables to increase the 4.pi.Is of M type ferrite by about 200 G at maximum, thereby achieving a Br of at least 4.5 kG. In this case, however, since the anisotropy field (HA) to be described later lowers, it is difficult to acquire a Br of at least 4.5 kG and a HcJ of at least 3.5 kOe at the same time.
HcJ is in proportion to the product (H.sub.A xfc) of the anisotropy field (H.sub.A =2K.sub.1 /Is) multiplied by a single magnetic domain grain fraction (fc). Herein, K.sub.1 is a constant of crystal magnetic anisotropy which is determined by the crystalline structure like Is. M type barium ferrite has a K.sub.1 =3.3.times.10.sup.6 erg/cm.sup.3, and M type strontium ferrite has a K.sub.1 =3.5.times.10.sup.6 erg/cm.sup.3. It is known that M type strontium ferrite has the highest K.sub.1 although it is difficult to achieve a further improvement in K.sub.1.
On the other hand, if ferrite particles assume a single magnetic domain state, the maximum HcJ is expectable because to reverse the magnetization, the magnetization must be rotated against the anisotropy field. In order that ferrite particles be single magnetic domain particles, the size of ferrite particles must be reduced to or below the critical diameter (dc) given by the following expression: EQU dc=2(k.multidot.Tc.multidot.K.sub.1 /a).sup.1/2 /Is.sup.2
Herein, k is the Boltzmann constant, Tc is a Curie temperature, and a is a distance between iron ions. Since M type strontium ferrite has a dc of about 1 .mu.m, it is necessary for the manufacture of sintered magnets, for example, to control the crystal grain size of a sintered body to 1 .mu.m or less. Although it was difficult in the prior art to realize such fine crystal grains at the same time as increasing the density and the degree of orientation for achieving higher Br, the inventors proposed a new preparation method in JP-A 53064/1994 and demonstrated the obtainment of better properties which were not found in the prior art. Even in this method, however, HcJ approximates to 4.0 kOe when Br is 4.4 kG, and it is difficult to simultaneously achieve a high HcJ of at least 4.5 kOe while maintaining a high Br of at least 4.4 kG.
In order to control the crystal grain size of a sintered body to 1 .mu.m or less, the particle size at the molding stage should preferably be controlled to 0.5 .mu.m or less when grain growth in the sintering stage is taken into account. The use of such fine particles generally gives rise to a productivity decline because the molding time is extended and more cracks occur upon molding. It is thus very difficult to find a compromise between property enhancement and high productivity.
Still further, it was known in the prior art that the addition of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 is effective for achieving high HcJ. In this case, Al.sup.3+ and Cr.sup.3+ are effective for increasing H.sub.A and restraining grain growth by substituting for Fe3+ having an "upward" spin in the M type structure, thereby achieving a high HcJ of at least 4.5 kOe. However, as Is lowers, the sintered density is also likely to decline, resulting in a significant decline of Br. Consequently, for compositions providing a HcJ of 4.5 kOe, only a Br of about 4.2 kG is available at the highest.
Meanwhile, conventional M type ferrite sintered magnets had a temperature dependency of HcJ of about +13 Oe/.degree. C. and a relatively high temperature coefficient of about +0.3 to +0.5%/.degree. C. This leads to a likelihood of demagnetization that HcJ experiences a great decline on the lower temperature side. To prevent such demagnetization, HcJ at room temperature must be as great as about 5 kOe, for example, and it is then substantially impossible to obtain high Br at the same time. The temperature dependency of HcJ of M type ferrite powder, which is superior to that of anisotropic sintered magnets, is still of the order of at least +8 Oe/.degree. C., and its temperature coefficient is at least +0.15%/.degree. C., and it is then difficult to further improve the temperature properties.
The inventors proposed in JP-A 53064/1994 that by pulverizing ferrite particles to introduce crystal strains, the temperature change rate of HcJ can be reduced. In this case, however, the HcJ of particles is also reduced at the same time. It is then difficult to acquire high HcJ and excellent temperature properties of HcJ at the same time even when M type strontium ferrite of submicron size is used.
Since ferrite magnets are well resistant to the surrounding environment and inexpensive too, they are often used in motors for use in various portions of automobiles. Since automobiles can be used in very cold or very hot environments, the motors are required to perform stable operation under such severe environments. Conventional ferrite magnets, however, have the problems that a substantial decline of coercivity occurs in a low-temperature environment as previously mentioned, and irreversible demagnetization called "low-temperature demagnetization" can occur.